Dense carbide composite for armor and abrasives

ABSTRACT

Hard, dense, composite ceramic bodies of boron carbide, silicon carbide and silicon, particularly useful as ceramic armor, are produced by forming a mixture of granular boron carbide and a temporary binder into a desired shape and setting the binder to obtain a coherent green body which is siliconized by heating it, in an inert atmosphere and in contact with a controlled amount of silicon, to a temperature above the melting point of silicon and in the range of about 1500*-2200*C, whereupon the molten silicon infiltrates the body and reacts with some of the boron carbide thereof.

United States Patent 1191 I Taylor et al.

1 1 Oct. 16, 1973 1 1 DENSE CARBIDE COMPOSITE FOR ARMOR AND ABRASIVES 73Assignee: The carborifiififii Company,

Niagara Falls, NY.

[22] Filed: May 22, 1967 [21] Appl. No.3 640,327

[52] US. Cl. 89/36 A, 106/44, 29/1827,

161/404, lO9/82, 5l/307 [51] Int. Cl. F4lh 5/00 [58] Field of Search109/82; 161/404;

[56] References Cited UNITED STATES PATENTS 1,703,416 2/1929 Donaldson109/82 w x 2,108,794 2/1938 Boyer et a1. 106/44 2,109,246 2/1938 Boyeret a1. 106/44 2,908,553 10/1959 Frank et al. 106/44 X 3,035,325 5/1962Nicholson et al.... 264/29 3,178,807 4/1965 Bergmann 29/1827 3,205,8419/1965 Shwayder 109/82 FOREIGN PATENTS OR APPLICATIONS 1,213,305 3/1966Germany 89/36 A OTHER PUBLICATIONS Refractory Hard Metals, Schwarzkopfet al., The Mac- Millan Co., New York, 1953.

Primary Examiner-Stephen J. Lechert, Jr. AttorneyK. W. Brownell [5 7]ABSTRACT Hard, dense, composite ceramic bodies of boron carbide, siliconcarbide and silicon, particularly useful as ceramic armor, are producedby forming a mixture of granular boron carbide and a temporary binderinto a desired shape and setting the binder to obtain a coherent greenbody which is siliconized by heating it, in an inert atmosphere and incontact with a controlled amount of silicon, to a temperature above themelting point of silicon and in the range of about l5002200C, whereuponthe molten silicon infiltrates the body and reacts with some of theboron carbide thereof.

8 Claims, No Drawings DENSE CARBIDE COMPOSITE FOR ABRASIVES BACKGROUNDOF THE INVENTION This invention relates to hard, dense, compositeceramic bodies of boron carbide silicon carbide and silicon; to methodsof making such bodies; and to such hard, dense boron carbide-siliconcarbide-silicon composite bodies which are particularly useful as armor.

There has been considerable effort devoted in recent years to thedevelopment of ceramic armor materials which are sufficiently strong toafford adequate protection against projectiles but which at the sametime are light enough to be useful in such applications as personnel andaircraft armor, where it is important that the weight of the amount ofarmor required for protection be at a minimum or at least withinpractical limits. It is also important that the processes employed inmaking ceramic armor materials be adaptable to the production of armorin rather intricate monolithic shapes such as helmets, vest sections,leg armor sections and the like.

Several comparatively lightweight ceramic materials have been reportedas useful for armor: sintered alumina; self-bonded silicon carbide; andhot pressed boron carbide. Of these, the alumina and silicon carbidematerials may be made by processes'which permit the formation of variousshapes, but they have relatively high specific gravities of the order of3.65 and 3.05, respectively, and effective amounts of these materialsarecomparatively heavy. Hot pressed boron carbide, on the other hand,has a relatively low specific gravity of about 2.452.5, but the natureof a hot pressing process is such as to render it difficult, if notimpossible, to form shapes other than flat plates and other relativelysimple shapes. Moreover, hot pressing is a particularly expensiveprocess and is not well suited to large scale production by continuousprocessing.

SUMMARY OF THE INVENTION The present invention contemplates a hard,dense (i.e. essentially nonporous) composite ceramic mate- ARMOR ANDrial consisting essentially of boron carbide, silicon carbide andsilicon, which has a relatively low specific gravity approaching that ofhot'pressed boron carbide, but which may be produced without resort tohot pressing by an economical method which lends itself readily to theproduction of bodies of various shapes. Accordingly, composite bodiesmay be produced in accordance with this invention which are especiallyuseful as ceramic armor. Such composite bodies may be quite suitable asarmor for protection against low caliber, low velocity projectiles, evenif they lack the optimum properties required for protection against highcaliber, high velocity projectiles. Composite bodies may also beprepared which, even if not especially useful as armor, nonetheless havedesirable properties such as hardness, wear resistance, strength andlack of porosity which render such bodies, in suitable shapes, useful asextrusion dies, sandblast nozzles, suction box covers for paper-makingmachines, buoyancy spheres and the like.

Briefly, the process of the invention comprises preparing an initialmixture of granular boron carbide and a temporary binder; forming theinitial mixture into a desired shape by pressing, extruding, investmentor slip casting or any other suitable method; setting the temporarybinder to impart sufficient coherence to the shaped green body to permitfurther processing; and siliconizing the coherent green body by heatingit, in an inert atmosphere which preferably is a vacuum and in contactwith a controlled amount of silicon, to a siliconizing temperature abovethe melting point of silicon and at least about l500-C but below about2200C. Thereupon, the molten silicon infiltrates the body and undergoesa rather complex reaction with some of the boron carbide, producing somesilicon carbide in situ; and to the extent that interstices exits in thebody between the remaining boron carbide and the newlyformed siliconcarbide, the interstitial space is permeated by free silicon.

DESCRIPTION OF PREFERRED EMBODIMENTS The invention will now be describedin detail and certain preferred features will be pointed out, partlywith reference to the examples which follow and which are intended toillustrate and not to limit the inventive conen EXAMPLE 1 A mixture ofgranular boron carbide of various grit sizes is prepared consisting of45 parts of 180 grit, 25 parts of 400 grit and 30 parts of 800 grit.Thereto is added 10 parts of an aqueous solution containing 10 percentpolyvinyl alcohol as a temporary binder, and the mixture is blendeduntil substantially homogeneous. The resulting mixture, or initialmixture, is screened through a coarse sieve to break up anyagglomerates. A 510 g quantity of the mix is placed in a steel mold 6.4inches square, (16.3 cmsquare) and pressed at 2700 psi kg/sq cm) to forma plate 6.4 inches square by 0.44 inch (1.12 cm) thick. The piece isremoved from the mold, placed in an oven at room temperature and heatedto 105C over a period of about 30 hours at a substantially constant rateof temperature increase to evaporate the water and set the polyvinylalcohol binder. The coherent green body thus obtained has a specificgravity of 1.55.

The green body is placed on a 7 inch square (17.7 cm square) piece ofloosely woven carbide cloth in the bottom of a dense graphite cruciblewhich is placed in a container disposed within the coils of an inductionfurnace. A 360 g quantity of granular silicon is carefully placed on topof the-green body to form a pile of generally pyramidal shape with thebase of the pile extending near the edges of the body.

The container is evacuated to a pressure of about 50 microns to providean inert atmosphere, and the power source to the induction coils isturned on. The reduced pressure is maintained throughout the heatingcycle. An optical pyrometer sighted on the piece is used to ascertainthe temperature of the piece as the temperature rises. At a temperatureof about 1400C the silicon melts, spreading out over the top of thepiece; and when a temperature of about 1600 C is reached, after a totalheating time of about 1.75 hours, the molten silicon infiltrates thepiece quite abruptly and reacts with some of the boron carbide toproduce silicon carbide. Thereupon, the power is immediately turned offand the furnace and its contents are allowed to cool to roomtemperature.

The ceramic plate thus produced has a specific gravity of 2.54, amodulus of rupture of 25,000 psi (1750 kg/sq cm) and a modulus ofelasticity of 49 X 10 psi (3.4 X 10 kg/sq cm), and is extremely hard andessentially nonporous. Anal.: total C, 14.18 percent; total Si, 41.79percent; total B (diff), 44.03 percent; free C, 0.07 percent; free Si,21.42 percent. X-ray diffraction analysis indicates that the bodyconsists of the following phases: a first boron carbide type, with adiffraction pattern corresponding to normal 8 C; a second boron carbidetype, with a diffraction pattern of boron carbide having an expandedlattice; alpha (hexagonal) silicon carbide; beta (cubic) siliconcarbide; and silicon.

The polyvinyl alcohol employed as the binder in Example 1 is dissipatedduring the siliconizing heating cycle, prior to the time that thesilicon melts, leaving no significant amount of carbon residue in thebody at the time the silicon infiltrates the body. For reasons whichwill be discussed hereinafter, however, it is often desired to have acertain amount of finely divided carbon present in the body at the timeof siliconization, in which case the silicon reacts not only with someof the boron carbide in the body, but with substantially all of thecarbon, thus producing silicon carbide from both of these carbonsources. One method of producing a body to be siliconized which containscarbon is to employ a temporary binder which is carbonizable, i.e. whichwill produce a carbon residue in the body upon heating. This method isillustrated in Examples 2 and 3. Alternatively, a quantity of finelydivided carbon of any suitable variety such as powdered graphite may beincorporated in the initial mixture. It is usually preferred, however,when the presence of carbon is desired in the body to be siliconized, toadopt both of the foregoing methods by incorporating carbon and acarbonizable binder in the initial mixture, as in Examples 4 and 5.

EXAMPLE 2 A mixture of granular boron carbide of various grit sizes isprepared consisting of 42 parts of 120 grit, 23 parts of 180 grit, 14parts of 400 grit and 14 parts of 800 grit. Thereto is added 7 parts ofa temporary carbonizable binder consisting of 53 percent of a liquidphenol-formaldehyde resin of the kind typified by that sold by VarcumChemical Division of Reichhold Chemicals, Inc. under the trade nameVarcum 8121 and 47 percent of furfural as a diluent therefor. Themixture is blended until substantially homogeneous and screened througha coarse sieve to break up any agglomerates.

A 495 g quantity of the screened mix is pressed at 3000 psi (210 kg/sqcm) to form a plate of the same dimensions as that prepared inExample 1. The piece is placed in an oven at room temperature and heatedto 150 "C over a period of about 24 hours at a substantially constantrate of temperature rise of about 5C/hr, to set and cure the resin, theresulting green body having a specific gravity of 1.58.

The body is then siliconized under substantially the same conditions andin the same manner as described in Example 1, employing 295 g of siliconand a vacuum of about 50 microns. As the temperature rises, but beforethe silicon melts, the binder carbonizes to produce carbon distributedthroughout the body, the amount of this residual carbon being about35-40 percent of the weight of the resin incorporated in the initialmixture. Upon infiltration of the piece by the molten silicon, thesilicon reacts with the residual carbon, as well as with some of theboron carbide, to produce silicon carbide.

The ceramic plate, or tile, thus produced has a specific gravity of2.55, a modulus of rupture of 25,300 psi (1770 kg/sq cm), and a modulusof elasticity of 49.5 X psi (3.5 X 10 kg/sq cm), and is extremely hardand essentially nonporous. Anal.: total C, 14.0 percent; total Si, 32.2percent; total B (diff), 53.8 percent; free C, 0.18 percent; free Si,21.0 percent.

EXAMPLE 3 A mixture of granular boron carbide is prepared consisting of18.7 parts of 16 grit, 23.4 parts of 36 grit, l 1.7 parts of 54 grit, l1.7 parts of 70 grit, 18.7 parts of 180 grit and 9.3 parts of 400 grit.Thereto is added 6.5 parts of the same carbonizable binder employed inExample 2 and the mixture is blended until substantially homogeneous andscreened through a coarse sieve to break up any agglomerates.Substantially the same procedure and conditions as in Example 2 areemployed to prepare and siliconize a green body from 520 g of the mix,

300 g of silicon being used. The resulting plate, of the same dimensionsas that prepared in Example 2, has a specific gravity of 2.51, a modulusof rupture of 12,000 psi (840 kg/sq cm) and a modulus of elasticity of44.5 X 10 psi (3.1 X 10 kg/sq cm), and is extremely hard and essentiallynonporous. Anal.: total C, 15.2 percent; total Si, 35.3 percent; total B(diff), 49.5 percent, free C, 0.35 percent; free Si, 20.0 percent.

EXAMPLE 4 A mixture is prepared consisting of 45 parts of grit boroncarbide, 23 parts of 180 grit boron carbide, 30 parts of 800 grit boroncarbide, 2 parts of powdered graphite with a particle size of about 50microns and smaller, and 9 parts of a carbonizable binder consisting of46 percent of a phenol-formaldehyde resin such as that used in Example 2and 54 percent of furfural as a diluent therefor. The mixture is blendeduntil substantially homogeneous and screened through a coarse sieve tobreak up any agglomerates. A 545 g quantity of the mix is pressed at2700 psi to form a plate having the same dimensions as that prepared inExample 1, and the plate is placed in an oven at room temperature andheated to C over a period of about 24 hours at a rate of temperaturerise of about 5C/hr, to set and cure the resin. The resulting green bodyhas a specific gravity of 1.73.

The piece is siliconized in accordance with the method and conditionsset forth in Example I, employing 282 g of silicon, the bindercarbonizing during the heating before the silicon melts. Uponinfiltration of the piece by the molten silicon, the silicon reacts withthe powdered graphite, the residual carbon from the binder, and some ofthe boron carbide, to produce silicon carbide.

The resulting plate has a specific gravity of 2.57, a modulus of ruptureof 34,700 psi (2400 kg/sq cm) and a modulus of elasticity of 51.2 X 10psi (3.6 X 10 kg/sq cm), and is extremely hard and essentiallynonporous. Anal.: total C, 16.5 percent; total Si, 32.6 percent, total B(diff), 50.9 percent; free C, 0.16 percent; free Si, 12.6 percent.

EXAMPLE 5 A mixture is prepared consisting of 40 parts of 120 grit boroncarbide, 30 parts of grit boron carbide, 10 parts of 400 grit boroncarbide, 20 parts of powdered graphite with a particle size of about 50microns and smaller, and 10 parts of the same carbonizable binderemployed in Example 4. The mixture is blended until substantiallyhomogeneous and screened through a coarse sieve to break up anyagglomerates. A 490 g quantity of the mix is pressed at 3000 psi to forma total C, 22.5 percent; total Si, 39.1 percent; total B (diff.), 34.8percent; free C, 0.71 percent; free Si, 69

percent.

Knoop 100 hardness determinations on the boron carbide, silicon carbideand silicon present in the bodies of this invention give values of about28503000, 2500-2700 and 700-1600'kg/mm sq, respectively, the value forsilicon being somewhat higher than the expected value of 700-900,possibly due to small amounts of boron being present in the silicon.Since the silicon phase is considerably softer than the boron carbideand silicon carbide, it is usually preferred to produce bodiescontaining the minimum amount of free silicon consistent with obtaininga sound, uncracked body, when hard bodies suitable for use as armor aredesired. Several interrelated process variables must be considered inthis connection, including the particle size of the boron carbide in theinitial mixture and the presence of carbon in the green body beingsiliconized.

The particle size of the granular boron carbide is convenientlyexpressed as grit size, each grit size number designating theapproximate range of particle sizes set forth in Table I.

TABLE I Typical Particle Size Distribution of Boron Carbide AbrasiveGrits Grit Microns:

No. Average MaximumMinimum l 6 1092 1650 7 87 36 483 762 305 S4 305 495203 70 203 330 127 i 20 102 165 50 1 80 76 1 14 25 400 22 45 1 1 800 1230 5 preferred, and is essential for producing highly effective armor,to employ a variety of boron carbide grit sizes in preparing the initialmixture, as in Examples 1-5, the variety being such as to permit densepacking and result in a green body having comparatively littleinterstitial space between the boron carbide particles. A combination ofabout 70 percent of relatively coarse grit and about 30 percent ofrelatively fine grit is usually suitable, at least as a starting pointfor further refinements which may be introduced on the basis ofexperimental determinations of the optimum combination for the intendedpurpose. Thus the granular boron carbide of Example 1 consists ofpercent relatively coarse 180 and 400) grit and 30 percent relativelyfine (800) grit; the granular boron carbide of Example 2 consists of 70percent and grit and 30 percent 400 and 800 grit; the granular boroncarbide of Example 3 consists of 70 percent 16, 36, 54 and 70 grit and30 percent 180 and 400 grit; and the granular boron carbide of Example 4consists of about 69 percent 120 and 180 grit and about 31 percent 800grit. It should be noted that the terms relatively coarse and relativelyfine refer to the relationship between the particle sizes present in agiven initial mixture, and any numerically designated grit size will berelatively coarse or relatively fine, depending upon the other gritsizes present.

From a standpoint of producing siliconized bodies of maximum strengthand hardness and which are highly effective as armor, it is essentialthat the granular boron carbide in the initial mixture have a maximumparticle size of about 300 microns or less, although coarser materialmay be employed to make composite bodies useful for less demandingpurposes. The maximum boron carbide particle size in Example 1 is about1 14 microns (180 grit) and most of the boron carbide is considerablyfiner, this mixture having the smallest maximum particle sizeillustrated by the examples; and plates produced as in Example 1 proveto be highly effective as armor when subjected to rigorous, controlledballistic tests which involve firing projectiles of selected sizes withcontrolled'velocites at the center of the plates. From a practicalstandpoint, however, there is a limit to how fine the boron carbidegrain may be, since as the particle size of the boron carbide isdecreased its reactivity with the silicon during the siliconizationincreases, and it becomes increasingly difficult to control thesiliconization and prevent cracking of the bodies which occurs,apparently as a result of a rapid and extensive reaction. In fact, it isdifficult to produce sound, intact bodies consistently when using the180400-800 grit mixture employed in Example 1. For this practicalreason, it is usually preferred to employ at least some granular boroncarbide which is coarser than 180 grit. While the use of such coarsermaterial tends to result in an increase in the volume of interstitialspace in the green body and therefore an increase in the amount of freesilicon in the siliconized body, this tendency twoard an increase in theamount of free silicon can be at least partially offset by the presenceof carbon in the green body at the time it is siliconized, which may beachieved by the incorporation of carbon and/or a carbonizable binder inthe initial mixture.

Comparing Examples 1 and 2 in this light, it is seen that the freesilicon content of the bodies produced is substantially the same,notwithstandingthe inclusion of some 120 grit boron carbide in theinitial mixture of Example 2, in which a carbonizable binder is alsoincorporated. As the green body of Example 2 is heated and the binder iscarbonized, carbon is produced in the interstices between theboron'carbide grains; and upon infiltration the molten silicon reactswith the carbon, as well as with the boron carbide, to produce siliconcarbide, thus reducing the volume of interstitial space available foroccupancy by free silicon. The body produced in Example 2 is comparableto that of Example l as armor in controlled ballistic tests, and themodulus of rupture and modulus of elasticity of the bodies of Examples 1and 2 are about the same.

Comparing Example 2 with Example 3, in which a substantial amount ofboron carbide grain larger than 300 microns is used in the initialmixture, it may be seen that the modulus of rupture and modulus ofelasticity are markedly lower for the body produced in Example 3although the free silicon content of the bodies of both examples isabout the same, a carbonizable binder being employed in both examples.Ballistic tests on bodies prepared as in Example 3 show that they areclearly inferior to bodies prepared as in Example 2, although useful asarmor for protection against low velocity, low caliber projectiles.Accordingly, it is usually preferred to exclude from the initial mixtureany substantial amount of boron carbide with a particle size of morethan about 300 microns.

Example 4 illustrates a particularly preferred and highly desirableinitial mixture for the production of composite bodies according to theinvention. It will be noted that the maximum particle size of thegranular boron carbide is well below 300 microns, but that some of theboron carbide is coarser than 180 grit. In addition to employing acarbonizable binder to provide for the presence of carbon in the greenbody at the time the silicon infiltrates, carbon in the form of powderedgraphite is incorporated in the initial mixture to aid in minimizing theamount of free silicon in the body after siliconization. The compositebody produced in Example 4 contains considerably less free silicon thanthe body of Example 1, and has superior mechanical properties. Ballistictests show that the composite body of Example 4 is as good as or betterthan that of Example 1 for use as armor, and the initial mixture ofExample tion. Yields of 80-90 percent are attainable using the 4 processof Example 4; Le. sound, crack-free siliconized bodies are obtained inabout eight or nine out of ten runs.

Considering Example 5, it is seen that by incorporating a comparativelylarge amount of carbon in the initial mixture, the amount of freesilicon in composite bodies of the invention may be reduced to a ratherlow level, 6.9 percent in this example. Bodies containing as little asabout 3 percent free silicon can be produced by suitably adjusting theboron carbide grit sizes and the amounts of carbon and carbonizablebinder in the initial mixture and optimizing the specific gravity of thecoherent green body. It will also be noted that the specific gravity ofthe composite body produced in Example 5 is somewhat higher than thespecific gravity of bodies of higher free silicon content producedaccording to Examples l-4. This is to be expected, since silicon has arelatively low specific gravity of 2.33 as compared to boron carbide andsilicon carbide with specific gravities of 2.5 and 3.2, respectively.

It is difficult, however, to consistently produce sound composite bodieshaving such a low content of free silicon unless the bodiesare-comparatively small, for reasons which are not entirely clear.Attempts to repeat Example 5 often result in cracked bodies, and theproblem becomes worse as the size of the body sought to be made isincreased. It may be conjectured that the cracking is due, at least inpart, to stresses developed as a result of the expansion which occurswhen the carbon reacts with silicon to form silicon carbide. ln anyevent, it appears that about 3 percent free silicon is the approximatepractically attainable minimum.

Even though composite bodies of such low silicon content may be producedand are highly effective as armor, the practical difficulty of achievinggood yields makes it commercially preferable to produce bodiescontaining at least about 10 percent silicon. Bodies prepared accordingto Example 4, which describes a more or less optimum compromise inrespect to the numerous variables involved in the process of theinvention, usually contain from about 12 percent to about 16 percentfree silicon.

Summarizing the foregoing, the initial mixture to be used in producingcomposite bodies of the invention may comprise granular boron carbide ofa single, uniform particle size but preferably contains boron carbidewith a variety of particle sizes, the variety preferably being such asto permit dense packing and leave a minimum of interstitial space whenthe initial mixture is formed into the desired shape. Further, it ispreferred that the boron carbide have a maximum particle size of about300 microns or less, and that it contain some grain which is coarserthan l grit. The initial mixture advantageously contains a carbonizabletemporary binder or carbon, and preferably contains both.

The temporary binder may be selected from among a wide variety ofmaterials recognized as suitable for such use, for example, polyethyleneglycols and methoxypolyethylene glycols such as those sold by UnionCarbide Corporation under the trade name CARBO- WAX, polyvinyl alcohol,and where a carbonizable binder is desired, phenolformaldehyde resins,epoxy resins, dextrine and the like.

The binder is employed in an amount sufficient to give an initialmixture which is of the proper consistency for forming into the desiredshape by the method to be employed. Even if the binder is of thecarbonizable type this amount is usually established without regard tothe amount of carbon present therein, since finely divided carbon can beincorporated in the initial mixture in an amount sufficient to providethe total quantity of carbon desired in the green body at the time ofsiliconization. However, it is usually preferred that the carbonizablebinder have a high carbon content so that as much of the carbon in thebody as possible comes from this source, such carbon generally beingmore finely divided and dispersed than the carbon added to the mix. Thecarbon added to the mix may be graphite, such as employed in theexamples, or another type of free carbon.

For any given initial mixture of granular boron carbide and binder, theoptimum amount of carbon to produce a composite body having the desiredproperties is best determined experimentally. Calculations based uponthe comparative specific gravities of the unsiliconized and siliconizedbodies and the computed interstitial volumes thereof do not afford ameaningful guide because the silicon reacts not only with the carbon,but with the boron carbide as well, and the extent of the latterreaction and effect thereof is not precisely predictable. Generally, theoptimum amount of carbon is that which will give good yields ofsiliconized bodies which are sound and not subject to cracking, and ofminimum free silicon content consistent therewith. An excess of carbontends to result in cracked bodies, presumably due to the expansionwhich-accompanies the conversion of carbon to silicon carbide, while aninsufficiency of carbon tends to result in a higher free silicon contentin the composite body. The graphite in the desirable initial mixture ofExample 4, present in an amount .of about 2 percent of boroncarbide-graphite portion of the mix, approaches the optimum amount forthe mix described, since an increase to 3 percent usually results incracking during Siliconization, while a decrease tends to result in ahigher free silicon content and correspondingly inferior properties inthe composite body.

After the ingredients of the initial mixture have been blended togetherand, if necessary, passed through a coarse screen to break up anyagglomerates, the mixture is formed into the desired shape and thebinder is set to produce a coherent green body. porosity. and theVarious methods may be employed to shape the mix. In general, themechanical properties of the composite bodies tend to improve as thespecific gravity of the green body is increased, and shaped mixes ofmaximum specific gravity and minimum interstitial space are thereforeusually preferred, subject, however, to the limitation that there mustbe a continuous intercon- Y nected network of interstitial space betweenthe boron carbide grains throughout the green body to permit theinfiltration of silicon. In practice this limitation is of littlesignificance because it would require extreme and specialized measuresto attain such close packing and high specific gravity in the shaped mixas to present 'a problem of insufficient porisity. Even when the initialmixture is pressed into the shape of a plate at pressures up to 3000 psias in the examples, the specific gravity of the green body is only about70 percent of the theoretical specific gravity, andthe green bodies aresufficiently porous.

When employing the desirable initial mixture of Example 4 to form flatplates by pressing, it is usually preferred to use a pressure of about2500-3000 psi, a green body having a specific gravity of about 1.70-1.76being obtained upon setting the binder. Pressures less than about 2500psi tend to result in a lower specific gravity, more porosity in thegreen body, and a higher silicon content after Siliconization, whilepressures considerably in excess of 3000 psi appear to give no furtherdensification. When other initial mixtures are employed, the optimumpressure varies to some extent with the particle size of the boroncarbide, somewhat less pressure being required to attain a givenspecific gravity as the particle size decreases. The amount of initialmixture employed will, of course, depend upon the volume and specificgravity of the shaped mix.

Although the examples illustrate shaping of the initial mixture only bypressing, various other methods such as extrusion, slip casting andinvestment casting may be used, and the method of choice will dependprimarily upon the shape desired. The composition of the initial mixturemay be varied, especially in respect of the binder, to obtain the mostsuitable mix for the particular method of forming employed.

For investment casting and slip casting a particularly desirable initialmixture is substantially the same as that described in Example 4 exceptfor the binder, and consists of 45- parts of 120 grit boron carbide, 23.parts of 180 grit boron carbide, 30 parts of 800 grit boron carbide, 2parts of powdered graphite, and 50 parts of a 4 percent solution ofdextrine in water. Preferably a small quantity of a viscosity-loweringagent is incorporated to render the mix more flowable, for example, apolyelectrolyte such as the one sold by R. T. Vanderbilt Company underthe trade name DARVAN 7. Casting techniques are well-known andaccordingly not described here in detail. In investment casting a porousmold with a cavity of suitable configuration to produce the desiredshape is filled with the mix, preferably with vibration to aid packingof the particles. The filled mold is allowed to stand until sufficientaqueous phase has been absorbed from the mixture into the porous mold topermit the body to be handled. The body may then be removed from themold and the binder set. Using the above-mentioned mixture, coherentgreen bodies having a preferred specific gravity of about 1.7 may beobtained by investment casting in intricate shapes such as helmets, legarmor sections, vest sections, etc., which bodies may then be heated tocarbonize the binder and siliconized to produce composite bodiesaccording to the invention which may be used as armor orfor numerousother purposes.

Another particularly useful embodiment of the invention involves formingthe initial mixture by extrusion. It is well-known that boron carbide isof limited utility as an abrasive because the grain tends to fractureconchoidally and thus become rounded and less abrasive as wear occursduring use. However, an initial mixture according to the presentinvention may be extruded to form a fine strand, which may be cut intoshort lengths and the binder therein set to produce grain which may besiliconized to obtain boron carbidesilicon carbide-silicon abrasivegrain. It has been found that such grain, even though containing asubstantial amount of boron carbide, nonetheless fractures in use insuch a way as to leave new, sharp, abrasive edges exposed, such graintherefore being well suited to use in abrasive articles.

When the mixture has been formed into the desired shape, the binder isset to obtain a coherent green body under conditions suited to theparticular binder employed. Usually a slow heating cycle such as used inthe examples is appropriate, the temperature being increased graduallyto permit dissipation of the volatiles without cracks forming in thebody. When a phenolformaldehyde or other resin is employed, setting thebinder may also involve curing the resin, whereas with solutions ofpolyvinyl alcohol, dextrine and the like, setting the binder primarilyinvolves evaporation of the solvent.

When a carbonizable binder is employed, the binder in the coherent greenbody may be carbonized by heating the body in an inert atmosphere to asufficiently high temperature to effect carbonization. Since volatilesare dissipated from the body during carbonization, it may be necessaryor desirable to control the carbinizing heating cycle by providing for aslow rate of temperature increase to permit the escape of volatileswithout cracking the body. The need for such control tends toincrease'with increasing size and thickness of the body. As may be seenfrom the examples, no specific temperature control is required under theconditions there set forth for bodies of the described dimensions; andwhile carbonization may, if desired, becarried out as a separate step,it may often conveniently be accomplished during the siliconizingheating cycle.

Siliconization of the green body is carried out by heating it, in aninert atmosphere and in contact with a controlled amount of silicon, toa siliconizing temperature above the melting point of silicon and atleast about 1500C, at which temperature the molten silicon infiltratesthe body and reacts with some of the boron carbide and withsubstantially all of the carbon present, if any, producing siliconcarbide in situ. The siliconization is conveniently carried out asdescribed in the examples, by placing the green body in a suitablecrucible in a suitable furnace with the requisite amount of granularsilicon spread on top of the body. A piece of carbon cloth, convenientlycarbonized burlap, may be placed between the crucible and the piece tobe siliconized to minimize any tendency for the piece to adhere to thecrucible.

The amount of silicon employed must be carefully controlled withinrather narrow limits to obtain sound, uncracked composite bodies havingoptimum properties for use as armor, this control being necessary toprevent the reaction of boron carbide and silicon from proceeding to anundesirable extent. Since the tendency for this reaction to proceeddepends largely upon the particle size of the boron carbide, the degreeof control over the amount of silicon must, in general, be greater asthe boron carbide particle size is decreased. However, since it isusually preferred to employ at least some rather fine boron carbide inthe mix in order to produce bodies of superior properties, the amount ofsilicon used must usually be carefully regulated.

The amount of silicon to be used to a given green body ordinarily cannotbe calculated with exactitude since it is difficult to predict how muchsilicon will react with the boron carbide and how much will be presentin the siliconized body as free silicon. However, a reasonably closeapproximation of the requisite amount of silicon can be made bysubtracting the measured specific gravity of the coherent green bodyfrom the approximate desired specific gravity of the siliconized bodyand multiplying by the volume of the body, thus computing the amount ofsilicon needed to give the desired weight increase, assuming as is truethat there is no appreciable change in the dimensions of the green bodyupon siliconization. If carbon cloth is employed, it will generallyreact stoichiometrically with the silicon to form silicon carbide, andthe additional amount of silicon required for this purpose can becalculated quite accurately. The precise optimum amount of silicon isbest determined experimentally for a green body of given dimensions andcomposition, using the calculated approximation as a starting pointwhich is subject to modification, to obtain a sound, essentiallynonporous composite body of the desired specific gravity. The body ispreferably free of any excess silicon on its surface, thus requiring nopolishing prior to use. Preferably there is no adhesion of the body tothe crucible, or to the carbon cloth if one is employed.

A small excess or deficiency of silicon can seriously affect the courseof the siliconization. Excess silicon gives rise to an increasedtendency of the body to crack, probably due at least in part to anunduly extensive reaction with the boron carbide, and possible due tocertain surface effects of the excess silicon resulting in a nonuniformbody having variable coefficients of thermal expansion. A deficiency ofsilicon results in a body of greater porosity, disadvantageous forarmor, and may result in a cracked body. In Example 2, where 295 g ofsilicon is employed, it is found that sound, un-

cracked bodies are produced only if the total amount of silicon is noless than about 270 g and no more than about 320 g.

It is desirable for the molten silicon to contact the piece as uniformlyas possible, since a substantial localized excess tends to result incracks at that location. Little difficulty is had in siliconizing asubstantially flat piece, the granular silicon merely being placed ontop of the piece where, upon reaching the melting point, it melts andcovers the surface. Bodies of more complex shapes such as cylinders maybe wrapped with a porous wick material such as carbon cloth, placed inthe crucible on another piece of carbon cloth, and surrounded with theproper amount of granular silicon so that upon melting the silicon isdrawn up by the wick, contacts the green body quite uniformly, andinfiltrates the body when a sufficiently high temperature is reached.

siliconization must be carried out in an inert atmosphere such as vacuumor an inert gas, e.g. argon, helium or the like. Nitrogen is notsuitable because it tends to react with the boron carbide at thetemperatures reached to form a coating of boron nitride on the piece,which coating impedes infiltration of the silicon. Vacuum is muchpreferred, and the higher the better, since it aids in the removal ofany air trapped within the piece. High vacuum permits infiltration ofthe silicon at temperatures as low as about 1500C, and a temperature ofabout I600C is required at a pressure of about 50 microns. In anatmosphere such as argon or helium at atmospheric pressure, a somewhathigher siliconizing temperature of about l850C is usually required, thishigher temperature being less desirable as a result of increasedreactivity of the boron carbide with concomitant greater difficulty incontrolling the siliconization to give a sound, uncracked compositebody. It is usually found that while inert atmospheres other than vacuumare suitable for smaller pieces, increasing difficulty is experienceddue to localized siliconization and cracking as the size of the body isincreased.

When heated to about 2200C, it appears that a phase change occurs in thebody, accompanied by melting and deformation. Furthermore, moltensilicon will tend to evaporate rapidly under vacuum at temperatures wellbelow 2200C. Thus 2200C represents the highest practicable temperaturefor siliconization, and preferably the lowest possible temperature isused in order to minimize the rate of reaction of the boron carbide withthe silicon.

It is apparent that various suitable types of furnaces may be usedinstead of the induction furnace of the examples. The rate oftemperature rise below the melting point of silicon is not especiallyimportant except as regards carbonization of the binder as discussedabove. When the silicon has melted, however, it is desirable that thetemperature increase as rapidly as possible to that at whichinfiltration occurs, to minimize any effects which the molten siliconmight have on the surface of the piece; and once infiltration hasoccurred, it is desirable to avoid any further temperature increase andto allow the piece to cool as rapidly as possible without causingcracking due to thermal shock to minimize the extent of the reactionbetween the boron carbide and silicon and thereby minimize the tendencyof the body to crack.

The composition and structure of the composite bodies of the inventionappear to be complex. X-ray diffraction analysis of the bodies producedin Examples 2-5 indicates that each body contains the same five phasesas reported for the body of Example 1, and bodies produced in accordancewith the invention invariably appear to consist essentially of boroncarbide, silicon carbide and free silicon. lt has been observed,however, that composite bodies siliconized at temperatures of about1850C usually have X-ray diffraction patterns which indicate that thebodies contain no beta-silicon carbide, only the alpha form beingpresent, whereas bodies siliconized at about 1600C usually appear tocontain both forms of silicon carbide. The two boron carbide typesinvariably appear to be present, both haw ing a boron carbide typerhombohedral structure, one of which types corresponds to 3 C, the otherhaving an expanded lattice and being of less determinate composition butcontaining at least boron and carbon and possibly containing somesilicon. There is some indication that the free silicon may, at least insome cases, contain minor amounts of boron. Minor impurities, such asiron and calcium which may be present in the granular silicon or boroncarbide employed, may also be present in the composite body.

The silicon carbide and boron carbide are present as mutually adherentgrains in the composite body, presumably as a result of the reactionwhereby silicon carbide is formed from the boron carbide. When freesilicon is present in amounts up to about percent, it appears as adiscontinuous phase, filling the interstices between the boron carbideand silicon carbide;'when present in amounts of about 15 percent ormore, it appears as a continuous phase which further bonds the boroncarbide and silicon carbide. It is usually found that the modulus ofrupture is somewhat lower in bodies containing more than about 15percent free silicon than in bodies containing less than about 15%thereof. As noted above, 3 percent silicon appears to be about theminimum attainable and amounts of at least about 10 percent arepreferred. While there is no specific upper limit to the free siliconcontent of the composite bodies, and bodies containing considerably morethan 35 percent are easily produced, it is usually preferred that thesilicon content be no more than about 25 percent if highly effectivearmor is desired. Bodies containing from about 10 percent to about 25percent of free silicon usually contain from about 50 percent to about80 percent boron carbide and from about 10 percent to about 25 percentsilicon carbide.

The composite bodies of the invention are essentially nonporous. Theirspecific gravity is generally within the range from about 2.5 to about2.75, but for armor it is usually preferred that the specific gravityrange from about 2.5 to about 2.6 since such composite bodies have theadvantage of being lighter in weight while equally effective as armor.The modulus of rupture may be as low as about 10,000 psi (700 kg/sq cm),especially when granular boron carbide with a particle size greater thanabout 300 microns is included in the mix,

but may be as high as about 37,000 psi (2600 kg/sq cm) or'more and ispreferably at least about 20,000 psi (1400 kg/sq cm) for use as armor.The modulus of elasticity usually ranges from a low of about 30 X lO psi(2.1 X 10 kg/sq cm), especially when granular boron carbide with aparticle size greater than about '300 microns is included in the mix, toa high of about 60 X 10 psi (4.2 X 10 kg/sq cm), and preferably is atleast about 45 X10 psi (3.1 X 10 kg/sq cm).

The composite bodies of the invention are virtually insoluble in diluteor concentrated hydrochloric, sulfuric or nitric acid, but may showslight weight loss in a hot 50 percent sodium hydroxide solution after24 hours. At l350C, bodies prepared in accordance with Example 4 showgood oxidation resistance and only about 10 percent reduction of theirmodulus of elasticity.

Except as otherwise specified, all references herein to percentages andto parts refer, respectively, to percentages and parts by weight; andreferences to microns or millimeters in describing pressure and vacuumrefer to microns or millimeters of mercury. Except as otherwisespecified, modulus of rupture determinations are made by the four pointloading system, and modulus of elasticity determinations are made by thesonic method.

' We claim:

l. A dense, composite ceramic body consisting essentially of from about50 percent to about percent boron carbide, from about 10 percent toabout'45 percent silicon carbide, and from about 3 percent to about 35percent free silicon.

2. A body as defined in claim 1 containing from about 10 percent toabout 25 percent free silicon.

3. A body as defined in claim 2 containing from about 10 percent toabout 25 percent silicon carbide.

4. A dense, composite ceramic body as defined in claim 1 which, by x-raydiffraction analysis, consists essentially of B,C, a boron carbide typehaving an expanded lattice, silicon carbide of the alpha form or thealpha and beta forms, and free silicon.

5. A body as defined in claim 4 wherein said silicon carbide is presentin an amount of from about 10 percent to about 25 percent and in boththe alpha and beta forms, and said free silicon is present in an amountfrom about 10 percent to about 25 percent.

6. Ceramic armor formed of bodies as defined in claim 5, said bodieshaving a specific gravity from about 2.5 to about 2.6 a modulus ofrupture from about 20,000 psi to more than about 37,000 psi, and amodulus of elasticity from about 45 X 10 psi to about 60 X 10 psi.

7. Abrasive grain consisting essentially of bodies as set forth in claim1, said bodies being granular.

8. Ceramic armor formed of bodies as defined in claim 1.

2. A body as defined in claim 1 containing from about 10 percent toabout 25 percent free silicon.
 3. A body as defined in claim 2containing from about 10 percent to about 25 percent silicon carbide. 4.A dense, composite ceramic body as defined in claim 1 which, by x-raydiffraction analysis, consists essentially of B4C, a boron carbide typehaving an expanded lattice, silicon carbide of the alpha form or thealpha and beta forms, and free silicon.
 5. A body as defined in claim 4wherein said silicon carbide is present in an amount of from about 10percent to about 25 percent and in both the alpha and beta forms, andsaid free silicon is present in an amount from about 10 percent to about25 percent.
 6. Ceramic armor formed of bodies as defined in claim 5,said bodies having a specific gravity from about 2.5 to about 2.6, amodulus of rupture from about 20,000 psi to more than about 37, 000 psi,and a modulus of elasticity from about 45 X 106 psi to about 60 X 106psi.
 7. Abrasive grain consisting essentially of bodies as set forth inclaim 1, said bodies being granular.
 8. Ceramic armor formed of bodiesas defined in claim 1.